1. Field of the Invention
This invention relates generally to calibration of a phased array antenna, and in particular to a method of determining calibration constants for calibrating a receive only phased array antenna.
2. Description of Related Art
A phased array radar requires a high quality antenna array calibration to achieve high accuracies for multiple target tracking and interception guidance. Calibration is typically performed in a Near Field Range (NFR) and then periodically verified with an external RF source. However, the NFR calibration works fine when the antenna array is physically small enough so that it can be placed in the range. If the antenna array is too large, use of NFR is not possible. Also, using an external source is not always practical depending where the radar is located.
Previous calibration methods include a random number of elements with known locations and orientations, and the polarization for the calibration signal is known or assumed. Also, it was assumed that the noise and multi-path would normalize to zero which could lead to a calibration bias if either one is correlated.
Often a large array is located in both noisy and multipath environments. It is desirable to perform calibration independent of receive array polarization and tolerant of failed receive elements.
Previous methods of phased array antenna calibration are described in the following patents:
U.S. Pat. No. 5,861,843 issued Jan. 19, 1999 to Ronald E. Sorace and assigned to Hughes Electronics Corp. of El Segundo, Calif. discloses an orthogonal phase sequence method of calibrating a phased array antenna whereby the phase of each element signal is sequentially switched once at a time through four orthogonal phase states. At each orthogonal phase state, the power of the array antenna signal is measured. A phase and an amplitude error for each of the element signals is determined based on the power of the array antenna signal at each of the four orthogonal phase states. The phase and amplitude of each of the element signals is then adjusted by the corresponding phase and amplitude errors. However, the calibration measurements must be made at precisely these four orthogonal angles and multiple transmit signals are sent out for each element resulting in sequential calibration.
U.S. Pat. No. 6,720,910 issued Apr. 13, 2004 to Jeffrey H. Sinsky, et al. and assigned to Lucent Technologies, Inc. discloses a phased array calibration using sparse arbitrarily spaced rotating electric vectors and a scale measurement system. Data points are generated by measuring the power of the transmitted/received signal while each antenna element is rotated through a series of phase angles. Thereafter, a pairwise comparison of the data points is performed in order to determine the phase and amplitude corrections needed for each antenna element. This pairwise analysis uses as few as three signal measurements for each antenna element, made at sparse, arbitrarily spaced phase angles. Further, the method includes smart data selection to ignore bad data points resulting from anomalies or noise bursts. However, this calibration method is for a transmit array, and requires four orthogonal calibration electric fields which are known and accurate. Pair-wise analysis uses at least three calibration signals for each element.
U.S. Pat. No. 5,477,229 issued Dec. 19, 1995 to Gérard Caille, et al. and assigned to Alcatel Espace of Courbevoie, France discloses a method of calibrating an active antenna which may receive or transmit. In the case of a receive antenna the signal on each radiating source is amplified and phase-shifted in a variable gain active module applying a variable phase-shift. The N amplified signals on the N channels are then combined by a power combiner and transferred over a single channel to a centralized receiver. However, this method of calibrating is performed in a near-field range with a near-field probe.